1. Field of the Invention
The present invention relates generally to methods and systems for providing spectral redundancy while modulating information to be transmitted with a single carrier signal.
2. Description of Related Art
Narrowband interference and deep spectral notches in the transmission channel of single-carrier modulation (SCM) systems are common impairments in high-speed digital transmission over twisted-pair subscriber lines, home phone-line networks, upstream transmission in CATV cable systems, and wireless communication systems. For example, these impairments can occur in Very-High Speed Digital Subscriber Line (VDSL) systems, which are currently in a definition phase. In a VDSL system, signals will be transmitted over twisted-pair subscriber lines in the frequency band from a few 100 kHz up to 20 MHz. Cable attenuation and crosstalk from other pairs in the same cable binder are the main impairments. Deep spectral notches in the channel transfer function may be caused by bridged taps. In addition, spectral notches may be intentionally introduced at the transmitter to prevent radiation from the cable into certain RF bands (such as amateur radio bands). Narrowband RF interference must be suppressed by notching the corresponding bands at the receiver. The characteristics of the narrowband impairments are often a priori unknown at the receiver and may change over time.
Adaptive decision-feedback equalization (DFE) is conventionally used to deal with these impairments. Good performance is achieved if interference levels and the depth of spectral notches are moderate, or the width of impaired spectral regions is small compared to the Nyquist bandwidth. However, if the impairments are more severe, DFE requires long feedforward filter and feedback filters, and system performance, as measured by the mean-square error, is generally degraded. Moreover, the coefficients of the feedback filter tend to become large and unending error propagation can occur.
This error propagation problem can be avoided by performing the feedback filtering operation together with modulo signal reductions in the transmitter, instead of the receiver. This so-called “precoding” technique allows obtaining a substantially intersymbol interference (ISI) free signal at the output of the feedforward equalizer in the receiver. Precoding also enables the use of trellis-coded modulation (TCM) or similar signal-space coding techniques on ISI channels. However, the capabilities of DFE and precoding are limited. If the impaired spectral regions are too wide, a SCM system must avoid these regions. Moreover, precoding requires sending the feedback filter coefficients from the receiver to the transmitter.
Thus, there is a need for SCM systems which can deliver practically ISI-free signals in spite of narrowband interference and deep spectral notches in the transmission channel, and which do not have the problems associated with DFE and precoding.